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Frequently Asked Questions

Angle Tuning Back to top...

AOI effect on RazorEdge spectraThe spectrum of any thin-film filter shifts toward shorter wavelengths when the angle of incidence (AOI) of light upon the filter is increased from 0° (normal incidence) to larger angles. In most cases, the filter spectrum becomes highly distorted at larger angles, and the shift can be significantly different for s- and p-polarized light. The spectral shift that you experience is highly dependent upon filter design. To learn more about how angle tuning affects the spectral performance of different Semrock filters, please review our technical note: Filter Spectra at Non-normal Angles of Incidence.

VersaChrome® Tunable Bandpass Filters

VersaChrome filters are a revolutionary new optical filter technology: thin-film filters that are tunable over a wide range of wavelengths by adjusting the angle of incidence with essentially no change in spectral performance. More information about tunable filters can be found in our technical library.

If you have further questions about a specific application, please contact technical support for assistance.


Try angle tuning yourself, by using MyLight™. Select your filter and click on the blue "MyLight" button above the spectra to model the theory data at whatever angle you desire.

AOI and CHA Back to top...

Angle of Incidence, or AOI, is the angle at which a collimated beam of light is incident on the filter's first surface, measured relative to the surface normal. Cone half-angle, or CHA, refers to the angular range associated with a non-collimated incident beam, and is measured from the AOI to the largest cone angle.

Angle of Incidence and Cone Half-angle


Changing the AOI or CHA of your incident light will change the spectral response of the filter. For more information on how different types of Semrock filters respond to changes in AOI, please review our technical note: Filter Spectra at Non-normal Angles of Incidence. The spectral response to CHA is dependent upon many factors, so please contact technical support to discuss your specific application.

The AOI and CHA ranges for which filter performance is guaranteed are listed in each filter's Specifications tab on the website. The AOI and CHA specifications given are exclusive; filters may not meet specification when incident light has non-zero value for CHA and AOI.

ASCII Data Back to top...

ASCII data is typically a text file with two columns. The first column is the wavelength (measured in nanometers) and the second column is the corresponding transmission value of the filter at that wavelength. The transmission value is either on a scale of 0 to 1 or of 0% to 100%. ASCII data is available to download for all filters on the Semrock website. Most filters show actual measured data, while a few filters have theoretical data listed. The legend indicates that the displayed data is either measured or theoretical.

When you are viewing a filter's details you will find a link to the ASCII data for that filter to the right of the graph. Click on ASCII Data to save a copy of the text file that can be used to graph the filter's spectra.



If you need theory data on a single filter, use the blue MyLight button found above the graph and model your theory data, as well as download your ASCII data.

Bandwidth Back to top...

Semrock uses a “manufacturable specification” approach to define the bandwidth of our BrightLine® bandpass filters. We believe this approach more accurately reflects the performance of the filter in an optical system.

As shown in the diagram, the filter spectrum (red line) must lie within the unshaded regions. The average transmission must exceed the specification Tavg (%) in the Transmission Region, which has a certain center wavelength (CWL) and a width called the Guaranteed Minimum Bandwidth (GMBW). The filter part number has the form FF01-{CWL}/{GMBW}.

The transmission must lie below the blocking level specifications (OD) in the Blocking Regions. The precise shape of the spectrum is unspecified in the Transition regions. However, typically the filter passband has a Full Width at Half Maximum (FWHM) that is about 1% of the CWL wider than the GMBW bandwidth,


So, for the example shown in the diagram, the FF01-520/35 filter has a GMBW of 35 nm and a FWHM of 35 nm + 1% of 520 nm, or 40 nm.




Clear Aperture Back to top...

While the optical coating extends to the edge of every Semrock filter, the clear aperture will be less than the transverse dimensions for each filter.

For unhoused, round filters, the clear aperture is guaranteed to be greater than 85% of the outer diameter. For square and rectangular parts, the guaranteed clear aperture will be greater than 80% of the filter's transverse dimensions and is elliptically shaped. The clear aperture of a housed filter is dependent on the size and thickness of the housing ring. All filter specifications are guaranteed only within the clear aperture region.

Specific information can be found on the Specifications Tab for each individual filter.

Housed filter showing clear aperature example

Custom Coating Back to top...

If you would like a quote on a custom filter, please fill out our Custom Filter Specification Form and email it to semrock@idexcorp.com.

Our high-volume, IBS sputtering technology provides unsurpassed performance, repeatability and value. Our designers will meet your optical filter needs and also add value in other areas, such as optical cell design and measurement platforms. From prototype through production, you can count on us for expert design advice, unsurpassed performance and rock-solid reliability. To learn more about how you can benefit from our experience, contact Semrock's technical support or call 866-SEMROCK to discuss all of the available options.

Orientation of Semrock Filters Back to top...

Because of the durability of Semrock filters, you can easily populate filter cubes, filter sliders, and filter wheels yourself without fear of damaging the filters. To maximize intended transmission and blocking and to minimize autofluorescence, filters must always be oriented so that light is incident on a specific surface of the filter. This note describes the correct orientation for the different filter types.

Orienting Housed Excitation and Emission Filters

Semrock exciter and emitter filters mounted in housings feature an alignment arrow on the housing; see the illustrations below.  Orient such a filter so that the arrow points in the direction of light propagation. For microscopes, the exciter filter arrow should point away from the light source and toward the dichroic beamsplitter, and the emitter filter arrow should point away from the dichroic beamsplitter and toward the eye, detector, or camera.

Dichroic beamsplitters are rarely mounted in housings. See below for guidance.


Orienting Dichroic Beamsplitters and Other Unhoused Optical Filters

Dichroic beamsplitters and other unhoused optical filters feature orientation marks that identify the coated surface upon which light must be incident. An orientation mark is placed either on the front surface of the filter, or on the edge of the filter as a caret (^) mark. The different types of orientation marks are shown in the following drawings along with the corresponding orientation guidance.

  • Semrock logo: The logo is on the surface facing the incident light.
  • Line: A short line is on the surface facing the incident light. The line may be easier to see if viewed at an oblique angle.
  • Dot: A small dot is on the surface facing the incident light. The dot may be easier to see if viewed at an oblique angle.
  • Caret: A caret on the edge of the filter points in the direction of light travel. When the viewer faces the surface that receives the incident light, the caret points away from the viewer.

Caution: A number of dichroic beamsplitters have coatings on both surfaces. Always use the above instructions to identify the coated surface that should face the incident light! If you encounter any ambiguity or difficulty, please contact Semrock for assistance in identifying the surface orientation.

Further Resources

Complete how-to instructions for microscope cube assembly are available in PDF format and video.  

Optical Density Back to top...

Optical Density (OD) is a convenient tool to describe the transmission of light through a highly blocking optical filter (when the transmission is extremely small). OD is defined as the negative of the logarithm (base 10) of the transmission, where the transmission varies between 0 and 1.

OD = – log 10 (T)
T = 10 -OD

Optical Density Noise Floor:

Through careful optimization of our measurement equipment and methods, Semrock is able to provide some of the best wide bandwidth Optical Density (OD) measurements possible. Below are typical OD noise floor limitations for most of the measurement data shown on this website.

  • For wavelengths between 320 and 1120 nm, transmission values near or below 3e-7 (Optical Density 6.5) are measurement noise limited.
  • For wavelengths < 320 nm and between 1120 - 1500 nm, transmission values near or below 3e-6 (Optical Density 5.5) are measurement noise limited.
  • For wavelengths > 1500 nm, transmission values near or below 1e-5 (Optical Density 5.0) are measurement noise limited.

Also, for some filters and/or some blocking wavelength ranges, measurements with a noise floor of only about OD 4 are shown.

transmission OD OD rules Optical Density graph
Part Numbering Back to top...

General product naming conventions are as follows:

GMBW = Guaranteed Minimum Bandwidth

Occasionally there will be an additional ending to the part number after the diameter/dimensions are called out. The addition of a "-D" indicates that a part is unmounted. In this case, the diameter of 25 means that diameter of the glass substrate is 25mm. The addition of a "-N" calls for a part to be unmarked and will not include the Semrock logo, part number, or arrow to indicate the proper orientation.

In the case of a multiband or multi-notch filter, the multiple bands will be called out in an xxx/yyy format, as in this multiband dichroic:

Product naming conventions dual bandpass


For those parts who have multiple grades available for purchase, the grade is specified in this way:

product naming conventions with grade



The following is a guide to help you correlate part numbers and product families. It is not intended for users to create part numbers.


AbbreviationStands For...Product Family
BLP Basic Long Pass EdgeBasic™
BSP Basic Short Pass EdgeBasic™
-D unhoused, has no metal ring All
Di Dichroic BrightLine®
E E-grade filter RazorEdge®, StopLine®
Em Emission BrightLine®
FDi Flattest dichroic, used for imaging applications BrightLine®
FF Fluorescence filter BrightLine®
FRET Fluorescence resonance energy transfer BrightLine®
Hg Periodic symbol for mercury MaxLamp®
LD Laser diode MaxDiode®
LF Laser fluorescence BrightLine®
LL Laser line MaxLine®
LM Laser mux LaserMUX
LP Long pass BrightLine®, RazorEdge®
LPD Long pass dichroic RazorEdge Dichroic
M Emitter BrightLine®
-N not marked All
NF Notch Filter StopLine®
NIR Near-Infrared Near-IR
PBP Polarizing bandpass Polarizing bandpass
QD Quantum Dot BrightLine®
R Reflected BrightLine®, RazorEdge®
S S-grade filter RazorEdge®, StopLine®
SDi Short-pass dichroic BrightLine®
SP Short pass RazorEdge®
-STR Sutter Threaded Ring All (≤ 2 mm substrate required)
T Transmitted BrightLine®
TBP Tunable bandpass VersaChrome®
U U-grade filter RazorEdge®, StopLine®
X Exciter BrightLine®
What is Pixel Shift Back to top...

Pixel shift results when a filter in an imaging path (the emitter and/or dichroic beamsplitter in a fluorescence microscope) deviates the light rays to cause a shift of the image detected on a high-resolution CCD camera.  This shift becomes problematic when two or more images of the same object are acquired using different filter sets and then overlaid in order to simultaneously view fluorescence from multiple fluorophores.   Images produced by different fluorophores (and different filter sets) will not be accurately correlated or combined because each image is shifted by a different amount according to the wedge angles found in each filter set.  To eliminate pixel shift, BrightLine ZEROTM filter sets are manufactured and tested to exacting tolerances to ensure accurate image registration when combining multiple images.

The BrightLine ZERO™ option guarantees that the worst-case image shift when interchanging Semrock ZERO sets will be less than ± 1 pixel, measured relative to the mean image position for a large sample of filter sets.

pixel shift 1

This schematic of a typical epifluorescence geometry (as in a standard microscope) shows how filter wedge causes pixel shift.

pixel shift 2
Polarization Back to top...

When light is incident on an optical filter at a non-normal angle of incidence, the polarization of the light can be described by two orthogonal vector components associated with the orientation of the electric field of the light wave. The polarization is referenced to the “plane of incidence,” or the plane that is parallel to the normal to the surface of the filter and contains both the incident and the reflected light rays. The polarization component that is perpendicular to the plane of incidence is called the “s” component, and the component that is parallel to the plane of incidence is called the “p” component.

Rayleigh Range Back to top...

The Rayleigh Range is the distance from the beam waist (smallest focused spot) to the point where the waist has increased by a factor of √2. Images focused within one Rayleigh Range of the waist appear to be in focus. When working with gaussian beams (often an excellent model for a laser beam), the depth of focus is quantified by the Rayleigh Range. In order for images to remain in focus, reflecting filters (dichroic beamsplitters, for example) need to be flat enough to cause the reflected beam to shift less than one Rayleigh Range.

rayleigh range formula

Where R is the radius of curvature of the filter, D is the diameter of a laser beam incident on the filter, and λ is the wavelength of interest. Semrock specifies the flatness of dichroics by listing the nominal radius of curvature and also the waist diameter of the beam that will maintain less than one Rayleigh Range of focal shift.

Please note: The flatness specifications for our dichroic beamsplitters are provided for unmounted parts. Normal filter mounting procedures (using metal housings and epoxy cements) impart stress on the dichroic substrate that will negatively affect flatness. Our FH1 filter holder was designed to hold unmounted filters, like dichroic beamsplitters, making them easy to attach and align to an optical table without imparting stress.

Substrate/Filter Thickness Back to top...

We can coat and cut on a wide range of substrate thicknesses.

Each housed filter has one standard thickness, either 3.5 mm or 5 mm, and is defined by the filter's aluminum housing (see diagram below). The thickness of an unmounted filter is defined by the thickness of the filter's glass substrate.

If you have further questions about filter housings or substrate thicknesses, you may contact Semrock's technical support.

Online information is located under the spectra graph for each part or set:

Sutter Threaded Rings Back to top...

Semrock has worked with Sutter Instruments to design and introduce an integrated filter wheel mounting ring for our optical filters that is threaded to match the openings in a Sutter filter wheel hub. With this inclusive mounting system, the optical filter and threaded retaining ring are incorporated into a single component (see Figure 1). Sutter filter wheels traditionally have been designed to accept optical filters up to 9 mm in thickness with each filter wheel position incorporating a mounting hardware system that consists of a filter cup and retaining ring. The Semrock/Sutter integrated threaded ring solution is designed to be installed after removing the filter cup and retaining ring. By eliminating the need for the cup and ring hardware, a Semrock optical filter and integrated threaded ring mount assembly actually weighs less than the cup and ring it replaces.

Figure 1:
Left: Traditional mounted optical filter. Right: Semrock/Sutter threaded ring mount


The Sutter threaded ring option is available for all Semrock “Pinkel” (single-band exciters with multiband emitter) and “Sedat” (single-band exciter and emitters) multicolor filter sets. The option is denoted by the suffix “-S01” appended to the filter set catalog part number and is accompanied by a nominal $20 price premium per excitation filter in “Pinkel” sets, or per excitaiton and emission filter in “Sedat” sets.

Individual catalog filters which have a substrate thickness ≤ 2 mm can be purchased with the Sutter threaded ring option. The option is denoted by the suffix “-STR” appended to the catalog part number and is accompanied by a nominal $20 price premium per filter.


Sutter threaded rings are laser marked with an arrow to indicate the Direction of Light Propagation (DLP).

• Excitation filters: the arrow points toward the shoulder, in the direction of the threads. When the filter is threaded into Sutter's Lambda 10 threaded filter wheel used in the excitation light path, the arrow points in the DLP toward the microscope.

• Emission filters: the arrow points away from the shoulder, in the direction of the two notches. When the filter is threaded into Sutter's Lambda 10 threaded filter wheel used in the emission light path, the arrow points in the DLP away from the microscope and towards the detector.

STR Arrow Orientation Exciter
Sutter Threaded Ring Arrow Orientation - Exciter
  STR Arrow Orientation Emitter
Sutter Threaded Ring Arrow Orientation - Emitter

For additional details on installing filters mounted in a Sutter Threaded Ring into a Sutter filter wheel, please read the Sutter Threaded Ring Installation Guide.

Wavenumbers Back to top...

The "color" of light is generally identified by the distribution of power or intensity as a function of wavelength. Sometimes it is convenient to describe light in terms of "wavenumbers," where the wavenumber (w) is simply equal to the inverse of the wavelength and is therefore proportional to frequency.

Wavenumbers are often used in Raman spectroscopy since the separation of a particular Raman line from the laser line is fixed by the molecular properties of the material and independent of which laser wavelength is used to excite the line. This means that the shift is of constant frequency, regardless of excitation wavelength and can be conveniently expressed in terms of wavenumbers.

wavenumber formula


To learn more, please review our technical note Measuring Light with Wavelengths and Wavenumbers.


Custom Size a Filter Back to top...

Semrock offers custom sizing of most catalog filters right on our website. Whether you need an unhoused / unmounted round or rectangular filter, or the filter mounted into one of Semrock’s standard-size aluminum housings, use our custom sizing tool to calculate the price for the quantity you require and part number and add it to your cart to purchase.

Most custom size filters are available in less than one week. Simply input your required diameter, rectangular dimensions or required housing diameter. Semrock can size to any diameter from 5 mm to 50 mm in whole 1 mm increments, along with the most common English sizes, 12.7 mm (1/2 inch), 25.4 mm (1 inch), and 50.8 mm (2 inches) in less than one week. Gotta have that 31.4mm unmounted diameter? We can accommodate that as well, our inside sales team will quote the lead time with your order acknowledgement (typically four weeks). Semrock also carries standard-size aluminum housings for the following dimensions: 12.5mm, 25mm, 25mm Sutter Threaded Rings, 32mm, and 50mm.

Each filter which is available for custom sizing lists the dimensional range which can be accommodated for that filter on the product page. The substrate thickness and tolerance will be the same as for the standard size part for the filter of interest, simply click on the Specifications tab for details.

Need a size outside of these limits, contact Semrock at Semrock@idexcorp.com or 1-866-Semrock to inquire.

Custom Sizing

Filter Effective Index Back to top...

What is the difference between the Semrock ‘Effective Index of Refraction’ neff also referred to as Filter Effective Index and the Index of Refraction n?

neff is a filter-specific value used to calculate the angle of incidence (AOI) dependent wavelength shift of spectral features. It is not related to the Index of Refraction n for the substrate that is coated to make the filter. For Fused Silica, n = 1.4570 at the specific wavelength of 632.8 nm. On the other hand, neff describes the entire filter, including the coatings on both sides of the filter as well as the substrate between those two coatings. Furthermore, neff differs from filter to filter, depending on filter design, even though all filters may be coated on the same substrate.

Filters have spectral features, e.g. a spectral edge at a particular wavelength that marks the transition between transmission and blocking. It is a characteristic of these filters that spectral features shift to lower wavelength with increasing AOI.

Because optical systems often generate light rays that are incident on filters at AOI > 0°, it is useful to be able to calculate how far the spectral features shift in wavelength with AOI. As described in the Technical Note Filter Spectra at Non-Normal Angles of Incidence, a spectral feature at wavelength λ0 shifts to wavelength λ at AOI = θ, according to a formula that contains neff:


FAQ questions Angle tuning VersaChrome MyLight AOI angle of incidence CHA cone half angle ASCII Bandwidth Clear-aperture CA Custom Coating Filter-thickness Thickness Orientation Arrow Arrow-direction Optical Density Pixel shift ZERO Pixel-Shift Polarization rayleigh Range Substrate thickness Sutter threaded rings Sutter-threaded-rings STR Wavenumbers Flatness Imaging Part Number OD Blocking GMBW FHWM Guaranteed Minimum Full-width-half-max Theoretical-data Measured-data Flatness-of-a-Dichroic-beamsplitter Pixelshift Orientation-of-a-filter Custom-size Sizing Custom-sizing Filter-sizing Effective Index of Refraction neff